1. Field of the Invention
The present invention relates to a gripping device attached to, for example, a leading end of an arm of an industrial robot so as to grip and assemble various components. More particularly, the present invention relates to a gripping device that detects the assembly reaction force during assembly of components and performs assembly while controlling the assembly force, and that is applicable to automated assembly with an industrial robot.
2. Description of the Related Art
In recent years, there is an increasing demand for automated manufacturing of products having a complicated structure, for example, cameras. For such products, it is necessary to perform high-speed and accurate assembly with a small industrial robot under fine force control.
Japanese Patent Laid-Open No. 61-241083 discloses a technique of controlling a robotic arm and a hand while detecting the assembly force with a displacement sensor provided between the robotic arm and the hand in order to accurately and reliably assemble a gripped component.
FIG. 7 shows a basic device configuration and a signal transmission system disclosed in the above publication. Referring to FIG. 7, a component “a” is gripped by fingers provided in a grip section G, and is assembled into a workpiece “b”. A driving unit, such as a motor, 23 operates a robotic arm A. To the driving unit 23, a control unit 24 inputs an operation signal and a position control signal for the robotic arm A.
A calculation unit 25 calculates the relative displacement between the robotic arm A and the grip section G in six axial directions on the basis of detection signals from displacement sensors provided in a sensor unit 10. The calculation unit 25 is provided with an allowance setting unit 27 for setting an allowance that allows the component “a” to be properly assembled into the workpiece “b”. A comparator 26 compares a displacement obtained from the calculation unit 25 and the set allowance, outputs the comparison result to the driving unit 23, and operates the robotic arm A so that the actual displacement is within the allowance.
FIG. 8 is a detailed view of the sensor unit 10, and FIG. 9 is a cross-sectional plan view, taken along line IX-IX of FIG. 8. Referring to FIG. 8, the sensor unit 10 includes an arm-side plate 11 to be attached to the leading end of the robotic arm A, a grip-section-side plate 12 to be attached to the grip section G, and elastic members 13 that couple and support the plates 11 and 12. The sensor unit 10 also includes a displacement detection mechanism provided between the plates 11 and 12. The displacement detection mechanism includes a beam 14 extending from the arm-side plate 11 toward the grip-section-side plate 12 and having a cross-shaped leading end portion.
As shown in FIG. 9, the cross-shaped leading end portion of the beam 14 has X-direction displacement sensors 15 and 16 provided on opposite side faces parallel to the Y-axis, and Y-direction displacement sensors 17 and 18 provided on opposite side faces parallel to the X-axis. On a surface of the cross-shaped leading end portion facing the grip-section-side plate 12, displacement sensors 19, 20, 21, and 22 are provided to face the grip-section-side plate 12 with a predetermined gap therebetween. The grip-section-side plate 12 is provided with protruding pieces 33, 34, 35, and 36 respectively facing the X-direction displacement sensors 15 and 16 and the Y-direction displacement sensors 17 and 18 with a predetermined gap therebetween.
With these structures, the robotic arm A is operated by the driving unit 23 according to a predetermined program input from the control unit 24 so that the component “a” gripped by the fingers of the grip section G is assembled (inserted) into the workpiece “b”. In this case, when there is a relative positional error between the component “a” and the workpiece “b”, the elastic members 13 are bent by the contact of the component “a” with the workpiece “b”, and a displacement of the grip section G relative to the robotic arm A is detected from the bending. By performing the inserting operation in a state in which the relative displacement is within a predetermined allowance, proper assembly is realized while preventing the component “a”, the workpiece “b”, and the gripping device from damage.
In the gripping device of the above related art, however, the sensor unit 10 is connected between the robotic arm A and the grip section G in series. When the sizes of the gripping device and the sensor unit 10 are reduced, it is difficult to ensure both a high force detection sensitivity of the sensor unit 10 and a high-speed operation, as follows.
The size of the sensor unit 10 in the longitudinal direction of the arm can be reduced only by shortening the distance between the arm-side plate 11 and the grip-section-side plate 12, because the elastic members 13 are provided therebetween. However, when the distance is shortened, the lengths of not only the elastic members 13 but also the beam 14 are reduced. Consequently, there is little distance between a support point of the beam 14 and the displacement sensors, and the amount of displacement detected by the detection sensors decreases. Hence, the detection sensitivity (the ratio of the displacement amount detected by the displacement sensors to the applied force) decreases.
On the other hand, the detection sensitivity can be increased by replacing the elastic members 13 with more flexible members so as to reduce the rigidity of the elastic members 13. Unfortunately, when the rigidity of the elastic members 13 decreases, the grip section G easily wobbles with respect to the robotic arm A. For this reason, during driving of the robotic arm A, it takes much time to stably position the gripping device.
This point will be described in detail. In the related art shown in FIG. 7, since the sensor unit 10 is connected in series between the robotic arm A and the grip section G, deformation fulcrums of the elastic members 13 in the sensor unit 10 are away from the center of gravity of the grip section G. Consequently, a moment produced by an inertial force due to the positional difference between the deformation fulcrums of the elastic members 13 and the center of gravity of the grip section G has a great influence on the grip section G during driving of the robotic arm A, and it also takes much time to stably position the sensor unit 10 connected to the grip section G. The time necessary for stable positioning increases as the rigidity of the elastic members 13 decreases.
When the gripping device of the related art is driven at a high speed in order to enhance the working efficiency, the above-described moment increases as the speed increases, and the time necessary for stable positioning increases. Although the rigidity of the elastic members needs to be increased in order to shorten the time for stable positioning, the increase in rigidity reduces detection sensitivity of the sensor unit, and makes it difficult to accurately detect the force.
In this way, in the gripping device of the related art, when the size of the sensor unit is reduced in the longitudinal direction of the robotic arm, it is difficult to ensure both a high detection sensitivity and a high operation speed.
The present invention provides a gripping device including a force sensor that achieves size reduction and speedup without decreasing detection sensitivity of a sensor unit.
The present invention also provides a gripping device incorporating a force sensor that minimizes the influence of an excessive moment on a sensor unit because of an inertial force during operation of a robotic arm, and that realizes a shorter positioning time and a higher operation speed.
The present invention further provides a gripping device including a force sensor that increases rigidity of elastic members, minimizes the difference in sensitivity between detection axes of the force sensor, and enables accurate force detection.